Ammonia Generation via N2 Reduction: Exploring the Role of Transition Metals Doped B12C6N6 Nanocage as Single Atom Catalyst

Abstract

Ammonia (NH₃) is a crucial chemical used in fertilizer production and is now being recognized as a carbon-free hydrogen transporter for renewable energy applications. Despite its significance, the commercial synthesis of NH₃ relies primarily on the century- old Haber-Bosch process, which consumes large amounts of fossil fuels and contributes significantly to global CO₂ emissions. As a result, establishing sustainable, low-energy routes for ammonia production under moderate settings has become a top research focus. The electrochemical nitrogen reduction reaction (eNRR) is a promising option, but its practical application is limited by slow N₂ activation kinetics, low Faradaic efficiency, and competition with the hydrogen evolution reaction (HER). The TM-doped B₁₂C₆N₆ nanocages as enhanced single-atom catalysts (SACs) for effective electrochemical NH₃ production are examined. In this study, density functional theory (DFT) simulations are performed to assess the structural integrity, thermodynamic stability, adsorption behavior, and electrical characteristics of different TM@B₁₂C₆N₆ combinations. Interaction energy is calculated to investigate the stability of all transition metal-doped boron carbon nitride (B₁₂C₆N₆) complexes, with the highest interaction energy observed for Sc@B₁₂C₆N₆. Moreover, electronic analyses, such as Frontier Molecular Orbital (FMO), Electron Density Difference (EDD) mapping, Natural Bond Orbital (NBO), and Quantum Theory of Atoms in Molecules (QTAIM), are used to investigate orbital interactions, charge transfer dynamics, and bonding characteristics during N₂ adsorption and reduction. The adsorption of molecular nitrogen on the TM@B₁₂C₆N₆ complexes exhibits negativex adsorption energy, confirming the exothermic nature of N2 adsorption. Among the screened candidates, the vanadium-doped nanocage (V@B₁₂C₆N₆) demonstrates outstanding catalytic performance, characterized by strong and favorable N₂ adsorption, effective electron donation/back-donation interactions, and robust bonding properties, as proven by QTAIM parameters. The EDD and NBO investigations show a considerable charge redistribution between the active site and the N₂ molecule, enhancing its activation. The predicted energy barrier for the potential-determining step (PDS) is -1.72 eV, indicating the catalyst's thermodynamic feasibility for efficient ammonia synthesis. This study emphasizes that the TM@B₁₂C₆N₆ complexes, i.e., V@B₁₂C₆N₆ system, serve as a viable SAC platform for sustainable ammonia production. Atomic-level insights guide the design of next-generation eNRR catalysts with higher selectivity and energy efficiency.